This invention relates generally to radio communication systems, and more particularly to a system for detecting when a communication device, such as a cordless telephone, is not capable of communication with a central station.
Examples of various radio frequency (RF), portable, or cordless communication systems include cellular telephones, trunking radios, and the next generation of cordless telephones (called CT-2, for cordless telephones, second generation). A cordless, or portable telephone is typically understood to be a transceiving device designed to travel with the user.
In these systems, to establish communication between the communication devices, such as cordless telephones, a requesting cordless telephone transmits a call request to a central or base station. Upon receipt of the request, the central station returns a grant code.
Generally, a communication channel comprises a pair of frequencies for duplex operation or a single frequency for simplex operations. An inbound frequency (on the simplex or duplex channel) carries information from the cordless telephones to the central station, while the outbound frequency (on the other frequency of the duplex channel or the same frequency on the simplex channel) carries information from the central station to the cordless telephones.
When a cordless telephone is operating at such a distance (or range) from the central station that the cordless telephone has insufficient power to transmit to the central station or the base, even though the more powerful central station's transmission can reach the cordless telephone, the cordless telephone, or handset, is not in communication with a central station. In this area of weak signal strength, the cordless telephone is out-of-range.
The out-of-range condition could also be caused by channel losses, such as those due to fading or other environmental conditions surrounding the handset. One major cause of additional channel loss is the collapsed state of the handset itself. When not in use, and as an aid to traveling, the handset will probably be carried in a collapsed or closed state, with its antenna retracted. For portability, the collapsed handset will most likely be carried in a briefcase, belt holster, or pocket. Depending on the material (i.e., metal) of the carrying receptacle, its location, and the handset's retracted antenna, may all result in additional losses on the channel.
Fortunately, these additional channel losses are not seen in normal operation of the handset, with the antenna extended. However, when the base detects an inbound call from a subscriber on a land-line telephone or a cordless telephone, for another subscriber, the base must signal the other subscriber's handset, while the handset is still in a collapsed state. If the base does not overcome the worsened channel condition, a reduced range for the handset would occur.
Adding a more robust type of signaling (having more probability of signal detection) or increasing the transmitted power during initial signaling can overcome the reduced sensitivity of the collapsed handset. However, both methods create another problem, if the handset is still out-of-range. Extra base transmission range will be gained, but the handset would ring, to announce an incoming call, without being in a location to establish a reliable communication contact. Thus, the handset could ring, but the data and/or voice communication link would not be present, until the user traveled back into range, to allow the handset's transmission to be received. This annoyance would also give the user a poor system operation perception. If the phone rings, the user should be able to answer it, and have its answer received. Therefore, there is a need to indicate an out-of-range condition, without increasing the perception of a poor system operation.